1. Field of the Invention
This invention relates to ion implantation apparatus for use in manufacturing semiconductors.
2. Description of the Prior Art
In ion implantation apparatus for use in manufacturing semiconductors, the impurities which are introduced into the silicon or gallium arsenide target are ionized in an ion source, extracted in the form of a beam, accelerated to a certain energy, and implanted into the target. At this time the concentration of impurity which is introduced into the target can be established exactly by measuring the current of the ion beam using a Faraday cup, and seeking, as the dose amount, the total number of particles of ions shown by the total of this current. In FIGS. 1 and 2 are shown, by way of example, a method of measuring ion beam current which is widely used at the present time in ion implantation apparatus. In the example shown in FIG. 1, the substrate (which may be a wafer) 1 is placed at the bottom of a Faraday cup 4, and the current in the incident ion beam 2 is measured by the current meter 5 which is connected to the Faraday cup 4. At this time a secondary electron suppressor 3, to which a negative voltage is applied from a suitable negative voltage power supply 6, is provided at the entrance of the Faraday cup 4 and prevents escape from the inside of the cup of the secondary electrons produced within the cup, thereby resulting in a tool which greatly reduces measurement errors. The example shown in FIG. 2 is a method disclosed in U.S. Pat. No. 4,234,797, wherein the substrates (or wafers) 12 are mounted on a disk 11 rotating at high speed. When the ion beam 13 passes through a slit 18 provided in this disk 11, the current is measured by a Faraday cup 15 provided behind the disk 11. As in the case of the example shown in FIG. 1, a current meter 16 is connected to the Faraday cup 15, and a secondary electron suppressor 14, to which a negative voltage is applied from a suitable negative voltage power supply 17, is provided at the entrance of the Faraday cup 15.
However, on substrates used in manufacturing semiconductor devices used in actual ion implantation apparatus, there are many cases in which photoresist is applied as a mask material. When the ions being implanted pass through this photoresist mask, part of the energy of the ions strikes the polymer forming the photoresist material, the molecular bonds thereof are broken, and hydrogen molecules (H.sub.2), nitrogen molecules (N.sub.2), carbon monoxide (CO), water (H.sub.2 O), carbon (C), nitrogen (N) etc. are released into the process chamber as outgases. The amount of gas released varies, depending upon the type of photoresist material, film thickness, energy and current of the ion beam, etc., but in the case of apparatus furnished with usual pumping speed, the vacuum in the process chamber can be 10.sup.-4 -10.sup.-3 Torr, which is even 2-3 orders of magnitude worse than normal vacuum. In this manner, in cases where the vacuum has become very much worse, the ions of the incident ion beam collide with these residual gases, some are neutralized, some lose electrons, and their charge state is changed, and the disadvantage results that the dosage amount can no longer be monitored exactly. Moreover, together with this disadvantage, large size, multi-unit vacuum pumps are introduced to increase pumping speed, and the increased size of the apparatus, and greater cost, invite further disadvantages. Again, if one compensates the beam current based upon the vacuum of the process chamber as described in U.S. Pat. No. 4,539,217, it is necessary to carry out vast numbers of preliminary tests concerning all related ion species, energy, gas species, and vacuum; and even if one does these tests provisionally, in actual fact the photoresist structure during ion implantation may be changed by radiation of the ions from time to time and moment by moment, and the vacuum in the process chamber, as well as the structure of the residual gases, may also change from time to time and moment by moment, so that it is very difficult to compensate exactly. This invention has the objective of overcoming these defects, and to provide a method of monitoring ion beam current which is practical and successful.